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By proposing a diffractive optical element having optical functions of a
random phase plate and a lens array in combination, and an illumination
apparatus employing this lens element, reduction of speckles, and
improvements in energy efficiency and light utilization efficiency are
achieved simultaneously.
In a transparent base material, by individually assigning or superimposing
an amount of variation in accordance with a random number onto each of
the depths of recessed portions constituting steps equivalent in value to
a lens or a lens array, recessed portions having irregular phase
variations are formed and a diffractive lens element (6) is made. Also,
in an illumination apparatus employing this diffractive lens element and
a laser light source, in order to obtain a uniform illumination light
from which speckles are eliminated, the diffractive lens element (6) is
rotated by a rotating means.

1. A diffractive lens element in which a cross-sectional form is formed in
a step shape by forming multiple recessed portions having different
depths with respect to a transparent base material, and has optical
functions of one lens or a plurality of lenses and a random phase plate
in combination, said diffractive lens element characterized in that
recessed portions having irregular phase variations are formed by adding
or superimposing a variation amount in accordance with a random number
onto depths of recessed portions constituting steps which are equivalent
in value to a lens or a lens array.

2. An illumination apparatus for obtaining a uniform illumination light
from which speckles are eliminated using the diffractive lens element
according to claim 1, characterized in that said illumination apparatus
is provided with a laser light source and rotating means for rotating
said diffractive lens element.

3. The illumination apparatus according to claim 2, said illumination
apparatus characterized in that a light from the laser light source is
irradiated on the diffractive lens element via a condenser lens after
being propagated by a optical fiber.

Description

TECHNICAL FIELD

[0001] The present invention relates to a technique for eliminating
speckles in an illumination apparatus using a coherent light source and a
phase type diffractive optical element.

BACKGROUND ART

[0002] With the miniaturization of semiconductors in recent years, a high
resolution is demanded of semiconductor inspection apparatuses using an
optical microscope. For such a purpose, two methods including NA
(Numerical Aperture) heightening and wave-length shortening are
conceivable, however, since an immersion objective lens cannot be used
for purposes of inspecting semiconductor apparatuses, there is imposed a
restriction such that "NA<1.0." As such, there is known an apparatus
which utilizes deep ultraviolet laser so as to achieve high resolution by
wave-length shortening, and which is made capable of achieving
approximately twice the resolution by observing an object with
approximately half the wave-length of visible light.

[0003] However, when using a laser as a light source, there are problems
in that speckle patterns (when a highly coherent light source is used and
the phase of the image forming light becomes disordered, an interference
pattern of an irregular form is superimposed on the image) occur in the
image, and a desired resolution cannot be achieved.

[0004] In order to eliminate the patterns mentioned above, methods
indicated below are known.

[0005] (1) A method in which a rotary diffusion plate is provided inside
an illumination optical system:

[0006] (2) A method in which a fiber-bundle (the difference in length is
made greater than the coherence length of the laser) is used for an
illumination optical system (for example, Japanese Laid-Open Patent
Publication No. HEI 6-167640).

[0007] However, in method (1) using a rotary diffusion plate, the
following problems are present.

[0008] Efficiency is not good because energy loss due to scattering and
reflection at the diffusion plate is large.

[0009] A large portion of the light is discarded and wasted, and the
efficiency in light utilization is low, because, since the radiance
becomes lower the greater the angle of emergence of the light from the
diffusion plate becomes, in an apparatus such as a microscope which
requires uniform light, only the light in a portion of a limited region
in which the angle of emergence is small contributes to the formation of
the image.

[0010] Further, in the case of method (2) mentioned above, for each of the
fibers, it is necessary to set a difference in length greater than the
coherence length of the laser, and as a result, the entire length of the
fiber bundle becomes extremely long. Therefore, energy loss becomes
prominent especially in the deep ultraviolet region having low
transmittance, because with respect to the light propagated inside the
fiber, it is attenuated in proportion with the square of the fiber
length.

[0011] Accordingly, the present invention makes it an issue to reduce
speckles and improve energy efficiency and light utilization efficiency
at the same time by proposing a diffractive optical element having the
optical functions of a random phase plate and a lens array, and an
illumination apparatus employing such an element.

[0013] FIG. 2 is a diagram illustrating an example of the form of a
micro-lens;

[0014] FIG. 3 is a diagram illustrating an example of the form of a random
phase plate;

[0015] FIG. 4 is a diagram illustrating an example of the form of a lens
element according to the present invention;

[0016] FIG. 5 is a diagram illustrating an example of the form of a
cross-section of a micro-lens;

[0017] FIG. 6 is a diagram illustrating an example of the form of a
cross-section of a random phase plate;

[0018] FIG. 7 is a diagram illustrating an example of the form of a
cross-section of a lens element according to the present invention;

[0019] FIG. 8 is a diagram illustrating an example of a configuration in
which an illumination apparatus according to the present invention is
applied to an optical microscope; and

[0020] FIG. 9 is a diagram illustrating a different example of a
configuration in which the illumination apparatus according to the
present invention is applied to an optical microscope.

DISCLOSURE OF THE INVENTION

[0021] In order to solve the issue mentioned above, the diffractive lens
element according to the present invention is one in which, in the
transparent base material, recessed portions having irregular phase
variations are formed by individually adding or superimposing a variation
amount in accordance with a random number to the depth of each of the
recessed portions constituting steps that are equivalent in value to a
lens or a lens array.

[0022] Further, in order to obtain a uniform illumination light from which
speckles are eliminated, the illumination apparatus according to the
present invention is one in which a laser light source and a rotating
means for rotating the diffractive lens element mentioned above is
provided.

[0023] Thus, according to the present invention, the diffractive lens
element also has the optical functions of a lens or a lens array, and a
random phase plate, and by rotating it, it is possible to suppress
speckle patterns, while also reducing energy loss and improving the
efficiency of light utilization, because there is no need to use a
diffusion plate.

BEST MODES FOR CARRYING OUT THE INVENTION

[0024] The present invention relates to a diffractive optical element and
an optical apparatus using such an optical element. In addition, the
diffractive lens is something which is drawing attention as an optical
element to replace conventional spherical lenses, and include, for
example, a binary phase type diffractive optical element.

[0025] FIG. 1A schematically indicates an example of the formation of a
2-stage level binary optical element. By covering a tabular transparent
base member 1 with a mask 2A, and performing an ion-etching treatment,
grooves or recessed portions 3, 3, . . . corresponding to the mask
pattern are formed. In addition, here, what is meant by 2-stage is that
it includes 2 conditions: a case in which recessed portions are formed,
and a case in which they are not. Accordingly, if levels of 4 stages are
set, as shown in FIG. 1B, four conditions including a case where a second
mask 2B is laid and the recessed portions are not formed (zero depth),
and 3 stages of depths become possible. Further, as shown in FIG. 1C, in
8 stages in which a third mask 2C is laid, 8 conditions including a depth
of zero become possible.

[0026] By proceeding with such operations, it can be seen that detailed
steps comprising 2 to the power n types of depths (including zero depth)
can be formed. In other words, by forming numerous recessed portions with
different depths in the transparent base member 1, the form of the cross
section is formed in a step-like shape, and an extremely precise element
having good diffraction efficiency can be fabricated (In particular, it
is suitable for manufacturing micro-optical elements).

[0027] In addition, if (the formed pattern of a Fresnel step form), of
which only the shape of the cross-section is shown in FIG. 1A through
FIG. 1C, has, for example, symmetry around the center axis of rotation of
the transparent base material 1, it can be seen that the form viewed from
the direction of this central axis (optical axis) makes a concentric
circle shape, and that it has lens functions comparable to a spherical
lens.

[0028] By employing the technology above, it becomes possible to replace
microscopic lens elements such as a micro-lens and diffractive optical
elements such as a random phase plate (an item which makes the phase of
the wavefront of an illumination light random such that it does not have
a uniform pattern), however, what becomes a problem here is rectilinear
light (zeroth order diffraction light). In other words, in a diffractive
optical element, due to the properties thereof, zeroth order diffraction
light does occur, however, this zeroth order diffraction light is
non-functional in terms of the optical functions as a diffractive optical
element.

[0029] Therefore, when employing a diffractive optical element, the
elimination of speckle patterns is accompanied by a problem (such as a
reduction in efficiency and an increase in the number of component parts
and cost) such that a need to take countermeasures such as eliminating
the zeroth order diffraction light with a spatial filter arises.

[0030] Accordingly, in the present invention, by using a diffractive lens
element having optical functions of both a lens or a lens array and a
random phase plate in one diffractive optical element, not only the
functions of a lens, but also the functions of a random phase plate, in
other words, irregular phase assignment are made use of, and as a result,
it is possible to realize the elimination of zeroth order diffraction
light and speckle patterns without employing a spatial filter and the
like.

[0031] FIG. 2 through FIG. 7 show in contrast the respective examples when
a micro-lens 4, a random phase plate 5, and a diffractive lens element 6
according to the present invention are made as diffractive optical
elements. FIG. 2 and FIG. 5, FIG. 3 and FIG. 6, and FIG. 4 and FIG. 7
respectively show the micro-lens, the random phase plate, and the
diffractive lens element according to the present invention. In order to
show the characteristics of the shape of each optical element in a manner
that is easy to understand, the diagrams shown in FIG. 2 through FIG. 4
express the image data indicating their shapes in 2-tone after a gray
scale conversion. FIG. 5 through FIG. 7 show the shape of a cross-section
(a step shape) at a plane surface including an optical axis or a base
axis.

[0032] FIG. 2 shows an example of the shape of the micro-lens 4
constituting a micro-lens array (an optical element having a
configuration in which micro-lenses are arranged in an orderly manner in
a two-dimensional array form), and has rotational symmetry around the
optical axis thereof. And as shown in FIG. 5, the shape of the
cross-section of the plane surface including the optical axis of this
lens takes on an orderly step shape.

[0033] As shown in FIG. 3, the random phase plate 5 has irregular recesses
and protrusions, and the shape of the cross-section thereof resembles
that shown in FIG. 6. In addition, such a shape is formed by sectioning
the surface of the transparent base member into a meshed form while also
irregularly varying the depth of the recessed portions by way of random
numbers.

[0034] As shown in FIG. 4, the diffractive lens element 6 has a form as
though irregular recesses and protrusions were added with respect to the
shape of the micro-lens 4. In other words, as shown in FIG. 7, although
it has step-like tendencies of the micro-lens 4 when viewed in
perspective, it has an irregular shape when viewed closely. In the phase
type diffractive optical element mentioned above, such a shape is formed
as recessed portions having irregular phase variations by individually
adding or superimposing a variation amount in accordance with a random
number onto the depth of each of the recessed portions constituting steps
having optical functions comparable to a lens.

[0035] For example, by individually assigning a variation amount with
respect to the depth of recessed portions generated by a random number
function (or a pseudo-random number function), it is possible to create
irregular phase variations.

[0036] In addition, in regard to the function as a random phase plate, in
the case that manufacture is difficult if completely random phase
variations are assigned by way of a random function, phase variations of
a plurality of stages within a phase range of 0.about.2.pi. may be
established, and be selected therefrom at random.

[0037] With respect to an illumination apparatus for obtaining a uniform
illumination light in which speckles are eliminated or reduced using an
optical element in which a plurality of such diffractive lens elements 6
are arranged on one sheet of transparent base material, a rotating means
for rotating the diffractive lens element is provided. In other words, by
rotating (for example, at a rotation rate of one hundred to ten several
hundred rpm) the diffractive lens element within a plane surface
perpendicular to the optical axis, it is possible to generate spatially
and temporally random phase variations, and it is possible to suppress
speckle patterns peculiar to coherent light. Further, since the separate
preparation of the micro-lens array and the random phase plate may be
dispensed with, system configuration is simplified, and it is also
advantageous in terms of lowering cost.

[0038] In addition, the illumination apparatus according to the present
invention is widely applicable to a variety of optical apparatuses
employing a coherent light source (a light source with high interference)
of a single wavelength, such as an optical microscope using multiple-mode
optical fibers, a pattern exposure apparatus, or an optical molding
apparatus, for example.

[0039] As an example of an application of the illumination apparatus
according to the present invention, FIG. 8 shows a configuration example
7 of a microscope employing a diffractive lens element, and is basically
said to be the configuration of Koehler illumination.

[0040] A laser light propagated through an optical fiber 9 from a laser
light source 8 of SHG (Second harmonic generation)-Ar laser and capable
of continuous oscillation, first becomes a parallel luminous flux by
being spread by a condenser lens 10, and is irradiated on a diffractive
lens element 11 (see FIG. 4 and FIG. 7 for the individual lens elements).

[0041] As indicated with arrows, the diffractive lens element 11 is such
that it is rotated around the central axis by a rotating means 12
including a motor or the like. Light transmitted through the diffractive
lens element 11 reaches a mirror 17 (a semi-transparent mirror) via a
lens 16 after going through an aperture stop 13, a lens 14, and a field
stop 15.

[0042] Then, the light irradiated on a sample object (TG) via an object
lens 18 is received by an imaging apparatus (for example, a CCD type
camera, a film type camera or the like) 20 via the mirror 17 and an image
forming lens 19.

[0043] According to the present configuration, by rotating the diffractive
lens element 11, it is possible to generate a random phase variation, and
speckle patterns peculiar to coherent light may be eliminated. In other
words, because the amount of light received is averaged and speckle
pattern noise is reduced by way of integration within an image capturing
period (or charge storing period) for an image pick-up element inside the
imaging apparatus 20 constituting an observation system or by way of
integration within an exposure period for a film type camera, it is
possible to increase the S/N (signal to noise) ratio.

[0044] In addition, when employing deep ultraviolet rays for the purpose
of shortening the wavelength, quartz may be conceived as a glass material
to be used in the diffractive lens element or the lens.

[0045] Further, although in the present example, one diffractive lens
element is used (for example, forming elements on both surfaces), various
embodiments, such as configuring an optical system in which a plurality
of diffractive lens elements are combined as deemed appropriate, and
rotating the entire optical system or a part thereof, are possible.

[0046] FIG. 9 shows a configuration example 21 of a microscope which uses
laser beams as they are, and the difference between FIG. 8 is that a
laser beam (LB) is directly irradiated with respect to a diffractive lens
element 11. In other words, if laser beams can be used as parallel beams
to begin with, the optical fiber 9 and the condenser lens 10 mentioned
above can be dispensed with.

[0047] In addition, various embodiments, such as the configuration of
transmission light type, not limited to those configurations indicated in
FIG. 8 and FIG. 9 are possible.

[0048] As is evident from what is described above, according to the
invention according to claim 1, because the optical functions of both a
lens or a lens array (two-dimensional array type) and a random phase
plate are provided in one optical element, it becomes unnecessary to
employ separate optical elements having each function.

[0049] Further, according to the invention according to claim 2 and claim
3, by rotating the diffractive lens element, speckle patterns can be
suppressed, while at the same time, because there is no need to employ a
diffusion plate, it is possible to achieve a reduction in energy loss,
and an improvement in light utilization efficiency.